Food in Rogue is scarce (and rumor has it that it is even more scarce on the way back up), so you need to move fast and keep moving. Waiting to constantly heal will burn up your food, leading to starvation. At the very least, you should wait until your are hungry before eating (possibly until you start getting weak, though this has a drawback of being close to fainting, which can be fatal if surrounded by monsters) - eating food when not hungry ends up wasting food potential.